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01/28/2015

Evolving Skulls of Seafood Enthusiasts

Smithsonian postdoctoral researcher Neil Kelley studied the skulls of marine mammals and reptiles to learn about the evolution of feeding strategies in these animals. Image by Nicholas Pyenson, Smithsonian Institution.

A friend and I were recently trying to decide on what to eat for lunch. He asked, “do you like sushi?” Is that even a question? I love sushi, good sushi. No cuisine provides a better opportunity to sample the great diversity of food from the sea. Of course, there is the fish: salmon, albacore, eel. But there is so much more: seaweed, fish eggs, crab, clam and even sea urchin if you are feeling adventurous.

As a Peter Buck postdoctoral research fellow in the Department of Paleobiology at Smithsonian’s National Museum of Natural History, I study the evolution of animals that have evolved a similar taste for seafood. Just like humans, many different groups of land animal have learned to take advantage of the ocean’s bounty. Some of these land animals have evolved to become dedicated ocean dwellers, giving rise to marine mammals–like whales, sea cows and seals–and marine reptiles–like sea turtles and marine iguanas.

Our modern oceans are not unique in hosting these interlopers. While dinosaurs ruled the land in the Mesozoic (about 250 to 65 million years ago), the oceans were colonized by animals with terrestrial origins: for example fish-shaped reptiles called ichthyosaurs, long-necked plesiosaurs, and gigantic swimming lizards called mosasaurs. I am especially interested how these animals, living and extinct groups descended from land-dwelling ancestors, were able to find a place in ocean food webs. Were they predators or herbivores? Did they eat everything in sight, or were they specialized to eat a narrow variety of foods?

Marine iguanas (Amblyrhynchus cristatus) and manatees (Trichechus manatus) are descended from land animals, but both have adapted to feed on ocean plants. Similarities in the skulls of these animals reflects this similar diet. Image by Donald E. Hurlbert, Smithsonian Institution.

To test the hypothesis that the anatomy of living marine mammals and reptiles is closely tied to their diet, I combed through scientific studies of the diets of 69 different marine species, including whales, seals, sea cows, sea turtles and others. I then took detailed measurements of the skulls and teeth from these species using the Smithsonian’s extensive collections of mammal and reptile skeletons. I also visited museums in California, including the California Academy of Sciences, UC Berkeley Museum of Vertebrate Zoology and UC Davis Museum of Wildlife and Fish Biology, and the Royal Ontario Museum in Toronto, Canada to expand my dataset.

Manatee. Image by Chip Clark, Smithsonian Institution.

I found that skull and tooth shape very closely match the known diets of living species. Also, species with similar diets–for example sea turtles, sea cows and marine iguanas, which all eat marine plants–show similarities in skull shape despite being descended from very different ancestors. However, within some groups, closely related species have evolved to take advantage of different resources. For example, crabeater seals in Antarctica have specialized teeth for straining krill, while closely related leopard seals have evolved massive skulls and sharp teeth allowing them to feed on a wide range of prey including fish, crustaceans, and penguins.

I am now applying these results to better understand the evolution of diet and feeding styles in extinct marine reptile groups. This work will help to reveal how forces such as climate change and mass extinctions have transformed ocean ecosystems through time, and created evolutionary opportunities for land animals with a taste for seafood.

You can read more about the ecology and evolution of marine mammals and marine reptiles in our paper published this week in the journal Biology Letters.

Editors note: Special thanks to Dr. Ryosuke Motani, Department of Earth and Planetary Sciences, University of California, Davis.

By Neil P. Kelley, Peter Buck postdoctoral research fellow, Department of Paleobiology, National Museum of Natural History